Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the
following provisions ( see General Instruction A.2. below):

¨

Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

On September 13, 2011, Pharmasset, Inc. issued a press release announcing that screening has begun in a Phase 2b, international study of PSI-7977 and PSI-938, two nucleotide analog polymerase
inhibitors for the treatment of chronic hepatitis C virus (HCV) infection. The QUANTUM trial will evaluate interferon-free regimens of PSI-7977 400mg QD and PSI-938 300mg QD with and without ribavirin over 12 or 24 weeks in patients with
HCV who have not been treated previously. The trial will also evaluate the use of PSI-938 as monotherapy. The full text of the press release issued in connection with the announcement is attached hereto as Exhibit 99.1 and incorporated by reference
into this Current Report on Form 8-K.

Item 9.01. Financial Statements and Exhibits.

(d)

Exhibits

99.1

Pharmasset, Inc. Press Release dated September 13, 2011.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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